1. Field of the Invention
The present invention relates to a power generating apparatus using a solid oxide fuel cell and, more particularly, to a power generating apparatus using a solid oxide fuel cell that comprises a cathode layer and an anode layer formed on a solid oxide substrate, and that can provide increased power generation efficiency and can easy utilization of heat, while also providing a compact construction, by employing a simple structure that does not require hermetic sealing.
2. Description of the Related Art
Heretofore, fuel cells have been developed and commercially implemented as a low-pollution power generating means to replace traditional power generation systems such as thermal power generation systems, or as electric energy sources for electric vehicles that replace internal combustion engines which use gasoline or the like as the fuel. Fuel cells are also attracting attention as power supply sources for personal computers, etc. For such fuel cells, much research work has been done to develop higher-efficiency and low-cost fuel cells.
There are various types of fuel cells classified according to how they generate power, one type being a fuel cell that uses a solid electrolyte. To give an example of such a solid oxide fuel cell using a solid electrolyte, there is a fuel cell that uses, as a solid oxide substrate, an oxygen ion conducting solid electrolyte formed from a calcined structure of yttria(Y2O3)-doped stabilized zirconia.
The fuel cell that uses such a solid oxide substrate comprises a cathode electrode layer formed on one surface of a flat plate solid oxide substrate and an anode electrode layer formed on the opposite surface thereof, and the solid oxide substrate, the cathode electrode layer, and the anode electrode layer together form one solid oxide fuel cell unit.
Oxygen or an oxygen-containing gas is supplied to the cathode electrode layer, while a fuel gas such as methane is supplied to the anode electrode layer. A potential difference occurs between the cathode electrode layer and the anode electrode layer and, when lead wires are connected to the cathode electrode layer and the anode electrode layer, the fuel cell can generate electric power.
However, in the case of a power generating apparatus using this type of fuel cell, separate chambers must be provided, one being an oxygen or an oxygen-containing gas supply chamber on the cathode layer side and the other a fuel gas supply chamber on the anode layer side.
On the other hand, in another type of fuel cell power generating apparatus, the fuel cell unit is constructed by forming a cathode electrode layer and an anode electrode layer on opposite surfaces of a solid oxide substrate, and an electromotive force is generated between the cathode electrode layer and the anode electrode layer by placing the fuel cell unit in a mixture gas atmosphere containing a fuel gas, for example, methane gas, and oxygen gas.
The principle of generating an electromotive force between the cathode electrode layer and the anode electrode layer in this type of fuel cell power generating apparatus is the same as that for the above-described separate-chamber type fuel cell power generating apparatus but, as the whole fuel cell unit can be placed in substantially the same atmosphere, the fuel cell unit can be constructed as a single-chamber type unit to which the mixture gas is supplied, and this serves to increase the durability of the fuel cell unit.
However, in this single-chamber fuel cell power generating apparatus, as the fuel cell has to be operated at a high temperature of about 1000° C., there is the danger that the mixture gas may explode. Here, if the oxygen concentration is reduced to a level lower than the ignitability limit to avoid such danger, there occurs the problem that carbonization of the fuel, such as methane, progresses and the fuel cell performance degrades. In view of this, there is proposed, for example, in Japanese Unexamined Patent Publication No. 2003-92124, a single-chamber fuel cell power generating apparatus that can use a mixture gas whose oxygen concentration is controlled so as to be able to prevent the progress of carbonization of the fuel, while at the same time, preventing an explosion of the mixture gas.
Another power generating apparatus that uses a solid oxide fuel cell is proposed in Japanese Unexamined Patent Publication No. 2003-297397. In this fuel cell power generating apparatus, the space inside the container, except the space occupied by the fuel cell unit, is provided with explosion preventing means, and further, the exhaust gas discharged from the fuel cell unit is burned by a combustion device to ensure safe treatment of the exhaust gas.
The above-cited Japanese Unexamined Patent Publication No. 2003-297397 further discloses a fuel cell power generating apparatus in which the solid oxide fuel cell unit is mounted in a vertical position within the container, and the container is changed from a hermitically sealed type to an open type.
In the fuel cell power generating apparatus changed to the open type, the exhaust gas discharged from the fuel cell unit is burned in the space created in the upper part of the container, thereby ensuring safe treatment of the fuel cell exhaust gas while prevention an explosion of the exhaust gas.
In this fuel cell power generating apparatus, it is possible to enhance the energy utilization efficiency by recovering part of the heat generated by the combustion of the exhaust gas by using a heat recovering means such as a heating coil provided in the combustion device. However, a heater is mounted around the outer circumference of the container to heat the solid oxide fuel cell unit. In view of this, the above-cited Japanese Unexamined Patent Publication No. 2003-297397 proposes that, in the fuel cell power generating apparatus, the solid oxide fuel cell unit be heated by the combustion device using the exhaust gas, thereby eliminating the need for the heater.
In this fuel cell power generating apparatus, the end of the exhaust pipe is pointed toward the sidewall of the container where the combustion device is mounted. The heat produced by burning the exhaust gas introduced to the end of the exhaust pipe is used for heating the fuel cell housing section of the container. The solid oxide fuel cell can thus be readily heated to its operating temperature.
However, in the fuel cell power generating apparatus disclosed in the above-cited Japanese Unexamined Patent Publication No. 2003-297397, the exhaust gas discharged from the solid oxide fuel cell unit is introduced to the outer circumference of the cylindrical container for combustion; the fuel cell unit itself can be heated by the combustion heat but, because the combustion of the exhaust gas occurs outside the container, a special exhaust pipe and a special combustion device must be provided and, in addition to that, the cylindrical container must be hermetically sealed, which makes it indispensable to provide filler layers. As a result, the fuel cell power generating apparatus not only becomes complex in construction but its volume also increases.
Accordingly, it is an object of the present invention to provide a solid oxide fuel cell that ensures safe exhaust gas treatment while preventing an explosion of the exhaust gas within the fuel cell, that allows the fuel cell unit to be readily heated to its operating temperature by the combustion of the exhaust gas, that provides a simple construction, and that has an increased the power generation density per unit volume.